Altered andesite (Phantom Cone Andesite, Middle Pleistocene, 346-403 ka; Phantom Ship Island, Crater Lake Caldera, Oregon, USA) 3 : 無料・フリー素材/写真
Altered andesite (Phantom Cone Andesite, Middle Pleistocene, 346-403 ka; Phantom Ship Island, Crater Lake Caldera, Oregon, USA) 3 / James St. John
| ライセンス | クリエイティブ・コモンズ 表示 2.1 |
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| 説明 | Crater Lake is a large, deep, freshwater lake in the Cascade Range of America's Pacific Northwest. It formed about 7,700 years ago when an ancient volcano called Mt. Mazama had an enormous explosive ash eruption. The eruption was followed by collapse of the mountain, leaving a large depression which later filled with water. Large holes or depressions formed when a volcano destroys itself or collapses are called calderas. Crater Lake Caldera in Oregon is a world-class example of this type of volcanic feature.------------------------------From park signage:At first glance, the cluster of dark, jagged rocks just off-shore calls to mind the image of a ghostly ship with tall masts and drooping sails. The "Phantom Ship" is actually a remnant of an ancient volcano called the Phantom Cone.Unlike the youthful Wizard Island made mostly of cinders erupted within the last few thousand years, the dense lavas of Phantom Ship may be more than 400,000 years old - the oldest exposed rocks in the Crater Lake Caldera.The ship consists of two overlapping andesitic lava flows, part of the layering within the Phantom Cone. The subsequent growth of Mt. Mazama - a much larger volcano - engulfed the Phantom Cone. After Mt. Mazama collapsed, Phantom Ship was exposed. It is part of a ridge that protrudes from the caldera wall. Most of the ridge is now submerged, leaving Phantom Ship stranded off shore.------------------------------Partial description from Bacon (2008):Andesite of Phantom Cone (Middle Pleistocene)Moderately porphyritic silicic andesite (60.5 - 61.5, rarely 63.5% SiO2) . . . lava flows . . . of Phantom Cone. Phantom Ship Island is composed of altered lava flows. Phenocrysts (10-20%): plagioclase (≤3.5 millimeters, rarely to 6 millimeters; most <2 millimeters), augite (≤1 millimeter), orthopyroxene (≤1.3 millimeters), and Fe-Ti oxide; abundant plagioclase microphenocrysts, commonly oriented, give rock a total crystal content of ~30 percent. Medium-grained plagioclase+augite+orthopyroxene±olivine aggregates common. Basaltic andesitic enclaves . . . in lava flows, generally altered dusky yellow. Medium- to dark-gray groundmass of dense rocks may be altered dark bluish gray or greenish black in hand specimen; plagioclase colorless or light olive brown; reddish- to grayish-brown Fe-oxide staining on joint surfaces and within rock; relatively porous . . . lava flow tops conspicuously altered to pale-olive, grayish yellow-green, dusky yellow, or light-olive-gray colors.K-Ar ages: 403±12 ka at lake level southeast of Phantom Ship; 346±20 ka at lake level at Kerr Notch.------------------------------Locality: Crater Lake Caldera (view from Phantom Ship overlook), Crater Lake National Park, southwestern Oregon, USA------------------------------Reference cited:Bacon (2008) - Geologic map of Mount Mazama and Crater Lake Caldera, Oregon. United State Geological Survey Scientific Investigations Map 2832 [accompanying pamphlet]. |
| 撮影日 | 2012-08-04 17:53:04 |
| 撮影者 | James St. John |
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